In telehandlers, the free end of the telescopic boom arm can be used to mount a variety of lifting implements (forks, shovels, hoists etc.). The boom arm is raised and lowered by means of a hydraulic cylinder and it can also be extended and retracted. The lifting cylinder needs itself to be connected pivotably to the vehicle body and to the boom arm, which makes the use of flexible hydraulic high pressure hoses inevitable. To safeguard against rupturing of the supply hose, it is normal to provide a safety check valve which prevents fluid from escaping from the hydraulic cylinder, and therefore collapse of the boom arm, if the high pressure supply hose should burst.
If the vehicle is driven on a road or over rough ground while carrying a load, the inability of the boom arm to move relative to the vehicle body creates a tendency for the chassis to oscillate and interferes with the handling and control of the vehicle.
U.S. Pat. No. 4,658,970, US 2005/0072144 and EP 1157973 all propose a “ride control” mode of operation, in which the lifting cylinder is connected to a hydraulic accumulator so that it acts as a spring. This allows sprung pivotal movement of the boom arm and thereby improves the drivability of the vehicle.
In US 2005/0072144, there is no safety check valve and rupturing of the supply hose to the lifting cylinder would result in an unsafe collapse of the book arm.
In EP 1157963 the accumulator is connected between the safety check valve and the working chamber of the lifting cylinder. This means that the line connecting the accumulator to the cylinder bypasses the safety check valve and if that line should rupture while the ride control is effective, the boom arm would be unsupported. If one instead connects the accumulator to the working chamber of the lifting cylinder through the check valve, as taught by U.S. Pat. No. 4,658,970, the operation of the check valve has to be overridden while the ride control is effective and, in this case, a rupture in either of the hoses connected to the lifting cylinder could cause the boom arm to collapse.